I want to measure audio frequency and ultrasonic in air and in water(under water), and then analyse the data. So the instrument must fit the following:
1. Frequency range: DC to 5 MHz or 10 MHz;
2. Frequency analyser(FFT Function);
3. Proper probes or sensors by which I can measure audio and ultrasonic frequency in water and under water.

With regard to sensors, you would need to source those yourself as they are specialised applications. In general though, they should be voltage output with BNC terminations (ideally). A web search should find what you require.